The answer, according to New York-based writer Mariana Gosnell, is in the way snowflakes form and fall to Earth. The process is detailed in her book Ice: the Nature, the History, and the Uses of an Astonishing Substance.

A snowflake begins to form when water vapor condenses around a speck of dust high in the clouds—up to six miles (ten kilometers) up—and then crystallizes.

How the water vapor keeps on condensing and where the snowflake falls "is what determines the way the snowflake, or snow crystal, looks when it lands on your coat sleeve," Gosnell said. "It is extremely sensitive to microenvironments."

Between 27ºF and 32ºF (-2.8ºC and 0ºC), for example, crystals take the form of six-sided plates. Below that, needles form. A few degrees colder yields hollow columns; chillier yet, fernlike stars.

If temperatures drop further, plates and columns form again.

These crystals—usually six-sided because of the way hydrogen atoms bond with oxygen to create water—may eventually sprout branches, which continue to grow as additional water molecules cluster on the crystals' surfaces. (See a snowflake photo gallery.)

Humidity also plays a role. Drier air encourages growth across flat surfaces, for example, while higher humidity encourages growth at the tips, edges, and corners. More water vapor also leads to faster-growing and more intricate crystals.

To further complicate matters, as a crystal falls, frost could freeze to it or another passing flake could break off some of the crystal's branches. Even the the approach of a water drop can influence how a branch grows.

"By the time it reaches Earth, it may not have any resemblance to the very simple crystal that it started out as six miles up," Gosnell said.